THE VENTRAL NERVOUS-SYSTEM DEFECTIVE GENE CONTROLS PRONEURAL GENE-EXPRESSION AT 2 DISTINCT STEPS DURING NEUROBLAST FORMATION IN DROSOPHILA

Within the Drosophila embryo, the formation of many neuroblasts depends on the functions of the proneural genes of the achaete-scute complex (AS-C): achaete (ac), scute (sc) and lethal of scute (l'sc), and the gene ventral nervous system defective (vnd). Here, we show that vnd controls neuroblast formation, in part, through its regulation of the proneural genes of the AS-C. vnd is absolutely required to activate ac, sc and l'sc gene expression in proneural clusters in specific domains along the medial column of the earliest arising neuroblasts. Using ac-lacZ reporter constructs, we determined that vnd controls proneural gene expression at two distinct steps during neuroblast formation through separable regulatory regions. First, vnd is required to activate proneural cluster formation within the medial column of every other neuroblast row through regulatory elements located 3' to ac; second, through a 5' regulatory region, vnd functions to increase or maintain proneural gene expression in the cell within the proneural cluster that normally becomes the neuroblast. By following neuroblast segregation in vad mutant embryos, we show that the neuroectoderm forms normally and that the defects in neuroblast formation are specific to particular proneural clusters.